i would take a good look at your rental agreement - if you moved in with gas bottle full (if it's taken a year to use it, i would assume this is the case) and you move out, it may be a condition of your agreement that the bottle be full on departing as you HAVE used the gas, and given it costs over $1.12 per litre (that was about 12 weeks ago and i know it's gone up since then), it's a HUGE amount of your bond that they can technically keep if the bottle is empty when you leave. not sure how big the bottle is at your house, but we had ours filled just recently (can't put my hands on the bill) but i know it was less than 150L of gas and we got a bill for over $200... that's not money you want to NOT be expecting to pay out...

realistically, when you think about it, you've used a product, you should be paying for it...